1. Field of the Invention
This invention relates to a transmission apparatus of the frame signal communication type carried on an orbiting satellite in a satellite communications system which employs a plurality of orbiting satellites.
2. Description of the Related Art
A multiple connection communications system wherein communications are performed by way of communications satellites normally employs geostationary satellites on a geostationary orbit. However, also another system wherein orbiting satellites are used for communications has been proposed since the period of the dawn of satellite communications in the decade of 1950.
In recent years, as increase of the size of a rocket for launching a satellite proceeds, the technique of launching a large satellite with a small satellite carried thereon has been established and a rocket for launching a small satellite has been developed, which allows economical launching of a small satellite. Incidentally, particular attention is paid to a satellite communications system based on orbiting satellites wherein a plurality of small satellites make a round-trip along a low earth orbit or a medium earth orbit. The satellite communications system is advantageous, when compared with another system which employs geostationary satellites, in that it involves less transmission delay and a transmission/reception terminal can be reduced in size. The "IRIDIUM system" proposed by Motorola, Inc., United States is known as one of such satellite communications systems based on orbiting satellites.
FIG. 7 shows a general construction of a satellite communications system based on low earth orbit satellites. Referring to FIG. 7, the satellite communications system shown includes a plurality of orbiting satellites 101 to 103 and so forth, and a plurality of network control stations 104 to 107 and so forth disposed in communication areas of the orbiting satellites 101 to 103 such that at least one network control station is present in each of the communication areas. Each of the network control stations 104 to 107 connects a communication channel of a satellite to a subscriber network 109, performs accounting management and performs management of network subscribers thereof. Among the network control stations 104 to 107, particularly the network 107 serves as a reference station for controlling the entire network. A small size communications terminal 108 may be carried freely by a subscriber of the network or carried in an automobile.
Each of the orbiting satellites 101 to 103 irradiates a plurality of spot beams in different radiation directions toward the ground from a transmission apparatus carried thereon to form cells from the individual spot beams on the ground surface. Therefore, satellite communications which make use of orbiting satellites are called satellite cellular communications. In each cell, several different frequencies are repetitively used to effect communications and to make effective use of frequencies. Further, the power dissipation is reduced by use of spot beams. It is to be noted here that the distance between each orbiting satellite and the central point of a cell is different among different cells.
FIGS. 8 and 9 illustrate a variation of a cell in the satellite communications system shown in FIG. 7 with respect to time. For example, the communications terminal 108 which has been positioned in the cell A2 of the orbiting satellite 102 as seen in FIG. 7 moves from the cell A2 so that it is positioned in the cell C1 of the different orbiting satellite 101 in FIG. 8 as a result of simultaneous movement of the orbiting satellites 101 to 103 and so forth, and then moves from the cell C1 so that it is positioned in the cell B1 of the same orbiting satellite 101 in FIG. 9 as a result of such simultaneous movement as described above. In this manner, the communications terminal 101 can always communicate by way of a cell.
In the satellite communications system based on low earth orbit satellites, since the orbiting satellites move relative to a communicating person, the distance between a orbiting satellite and the communications terminal varies momentarily during communications, and also the propagation delay of a signal varies incidentally. Further, also when the communications terminal moves, the cell to which the communications terminal belongs changes over between different cells. However, this can be considered equivalent to the fact that the orbiting satellites move relative to the communications terminal.
In a multiple connection communications system, it is normally required that ground stations which participate in the system, that is, communications terminals, be all synchronized with the same frames in order for them to receive various control signals or make timings for transmission signals. To this end, a method is adopted wherein the communications terminals which participate in the system receive a common channel and detect, from within the channel, a frame signal called unique word inserted for each frame period and then establish synchronization using such detection signal.
FIG. 10 shows a general construction of an exemplary reception frame synchronization circuit. A clock signal generated in the communications terminal is counted by a counter 110, and the count value of the counter 110 is compared with a reference value by a comparator 111. The counter 110 is reset in response to a detection timing of a frame signal detected from the reception signal so that a frame signal synchronized with a coincidence pulse outputted from the comparator 111 is produced.
With the conventional transmission apparatus of an orbiting satellite, however, when it is tried to continue communications in the satellite communications system based on such low earth orbit satellites as described above, since the orbiting satellites are always moving relative to a communicating person, the distance between the orbiting satellite and the communications terminal varies with respect to time and the propagation delay of the frame signal varies during communications.
FIG. 11 shows that the cell successively changes over like A, B, . . . , G relative to a communications terminal on the ground. It can be seen from FIG. 11 that the distance between the communications terminal and the orbiting satellite varies. FIG. 12 illustrates various timings when, although the distance varies in this manner, an adjustment of the delay time is not performed for each spot beam with respect to a frame signal sent out from orbiting satellite. Referring to FIG. 12, the frame number (1) indicates that a frame signal sent out by a spot beam which makes a cell A is received; the frame number (2) indicates that another frame number sent out by a spot beam which makes another cell B; . . . ; and similarly, the frame number (7) indicates that a further frame signal sent out by a spot beam which makes a cell G is received. When the distance between the orbiting satellite and the communications terminal on the ground varies with respect to time, the frame signal received and a frame produced in the communications terminal itself are displaced by a time corresponding to a difference in propagation delay amount.
In particular, while a plurality of spot beams are radiated toward the ground from an orbiting satellite, since the radiation directions of the spot beams are different from one another, the distance to the central point of a cell is different depending upon the magnitude of the radiation angle of the spot beam, and as the radiation angle increases, the distance increases. Accordingly, if an orbiting satellite transmits a frame signal merely as it is with a plurality of spot beams irrespective of the distances to the central points of the cells, since the propagation delay of the frame signal varies by variation of the distance between the orbiting satellite and the communications satellite, the frame signal received and the frame signal produced by a counter in the communications terminal as described hereinabove are displaced progressively from each other as time passes so that frame synchronization cannot finally be established appropriately any more in the communications terminal.